San Juan River Recovery Implementation Program Summary Report 1991-97

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1988 (Platania 1990). Adult razorback sucker have been collected from shoreline habitats in the San Juan Arm of Lake Powell. To study razorback sucker habitat use in the San Juan River, on March 22, 1994, the Program stocked five radio - tagged (6 -month duration tags) subadult (mean TL = 277 mm) razorback suckers (reared from San Juan Arm of Lake Powell brood stock) into the river at Bluff (RM 79.6), five at the Four Corners Bridge (RM 119.2), and five above the Mixer in New Mexico (RM 136.6) (Ryden and Pfeifer 1995b). Six of these fish were monitored during the remainder of 1994, and the other fish were monitored for shorter periods of time. Similar to habitat use in the Green River System, the radio - tagged razorback sucker used primarily slow water habitats in March and April, including eddies, pools, backwaters, and shoals that were typically warmer than the mainchannel habitats (Ryden and Pfeifer 1995b). During June and August, mainchannel fast water run habitats were used the most by the radio - tagged razorback sucker. This information indicated that subadult razorback sucker stocked in the San Juan River selected habitats on a seasonal basis that were very similar to habitats selected by adults in the Green River (Tyus 1987). Ryden and Pfeifer (1995b) indicated that the slow water habitats used in the spring were relatively rare habitats in the river, whereas the runs used in the summer were fairly common habitats. On October 27, 1994, 16 additional razorback sucker equipped with 2 -year radio -tags were stocked at the same sites as the spring 1994 stocking (Ryden and Pfeifer 1996b). These fish were of the same cohort as the fish stocked in March, but were larger, averaging 403 mm TL. In September 1995, 16 additional razorback sucker were radio - tagged and stocked in the same locations in the San Juan River. Information on habitat use obtained through 46 contacts with radio - tagged fish during 1995. Unlike the spring 1994 samples, razorback sucker in spring 1995 primarily used mainstream run habitat rather than slow water habitats. These habitats were the same temperature as most of the remainder of the river. Runs continued to be the favored habitat throughout the remainder of 1995, with only June observations showing significant use of low velocity habitats. Reasons for the apparent change in preferred habitat between 1994 and 1995 are not known but may be related to differences in size of the fish and differences in flow during these 2 years. As part of integration efforts for the 7 -year research effort, habitat preference information of radio - tagged, stocked, adult razorback suckers have been developed by Ryden (1997b) by comparing habitats used to habitats available in the vicinity of the fish. These data were presented by season, month, and by geomorphic reach to aid in the development of flow recommendations for this species. By season, the data indicated that radio - tagged razorback suckers used a variety of habitats but showed some preference for pools and eddies in spring (March, April, and May), for eddies in summer (June, July, and August), used runs exclusively in fall (September, October, and November), and preferred edge pools during winter (December, January, and February). Because relatively few young razorback sucker have been captured, habitat requirements are difficult to determine. Recent theory is that larval razorback sucker use warm, flooded bottomlands during spring runoff. Relatively high numbers of YOY razorback sucker have been found in such habitats along the Green River following high spring flows (Modde 1996). Reduction of spring flooding due to dams has reduced the availability of this habitat type and may be the reason so few young razorback sucker have been found during intensive sampling efforts in recent years. The few juveniles that have been collected from the mainstream river have generally been found in backwaters (Minckley et al. 1991). San Juan River Recovery Implementation Program December 1997 Summary Report 21 PR -602 -1